April 30 (Reuters) - A U.S. congressman is calling for a probe into the activities of armed militiamen who are supporting a Nevada cattleman in a high profile showdown with federal authorities over grazing rights, citing allegations they set up armed checkpoints on local roads.

Rancher Cliven Bundy of Bunkerville became a symbol for conservative Republicans in April, particularly among the Tea Party movement, for his longstanding defiance of court orders to remove his cattle from federal land.

The dispute led to a showdown this month in which the federal Bureau of Land Management sent in helicopters and wranglers on horseback to seize his cattle, before backing down for safety reasons as militias flocked to support the rancher.

Bundy's supporters hailed the outcome as a victory over government tyranny. Detractors called Bundy an outlaw, and conservatives later cooled toward the rancher after he made a series of racist remarks and suggested that African Americans may have fared better under slavery.

Camouflage-clad volunteer militia members have remained a visible presence in Bunkerville, about 80 miles northeast of Las Vegas, in the two weeks since the showdown despite the departure of Bureau of Land Management officers.

U.S. Representative Steven Horsford of Las Vegas, in a letter sent on Sunday, asked the Clark County sheriff to look into complaints that militiamen had been present in Bunkerville-area schools and churches and had set up armed checkpoints on state, federal and county roads, seeking proof of residency from motorists.

"We must respect individual constitutional liberties, but residents of and visitors to Clark County should not be expected to live under the persistent watch of an armed militia," wrote Horsford, a Democrat. "Residents have expressed their desire to see these groups leave their community."

A spokesman for Clark County Sheriff Doug Gillespie confirmed receipt of the letter on Wednesday and said Las Vegas police would investigate if the department is "confronted with a victim of a crime."

Ammon Bundy, the rancher's son, said armed militiamen do accompany his father to news conferences at a roadside protest area and were guarding the family home, but denied they had established a deliberately intimidating presence.

"They have sidearms, but they are not carrying rifles," he told Reuters. "But going down to churches and schools, or stopping people on the road, that's not happening."

The dispute between the Bundys and the federal government dates back to 1993 when the cattleman stopped paying monthly fees the government charges ranchers to allow their cattle to roam federal range lands.

The government says Bundy, who does not recognize the federal government's authority over the land, owes more than $1 million in unpaid grazing fees and had ignored court orders to remove his cattle from public land.

It's not immediately clear how long the militia might stay in Bunkerville, although the younger Bundy said he thought it could stretch several months and possibly years, as the dispute proceeds.

"This kind of armed presence and force does help keep the peace," he said. (Editing by Cynthia Johnston and Diane Craft)